eth/downloader: allow all timers to exit

[holiman]

There's an issue in statesync, which can cause the spindown to become blocked. Example trace:

goroutine 61 [select, 25 minutes]:
github.com/ethereum/go-ethereum/eth/downloader.(*Downloader).spindownStateSync(0xc003576000, 0xc011a80c88, 0xc050990000, 0xf3, 0x100, 0xc15126c3c0, 0xc0ee278ae0)
        github.com/ethereum/go-ethereum/eth/downloader/statesync.go:226 +0x298
github.com/ethereum/go-ethereum/eth/downloader.(*Downloader).runStateSync(0xc003576000, 0xc125033f40, 0x0)
        github.com/ethereum/go-ethereum/eth/downloader/statesync.go:131 +0x1002
github.com/ethereum/go-ethereum/eth/downloader.(*Downloader).stateFetcher(0xc003576000)
        github.com/ethereum/go-ethereum/eth/downloader/statesync.go:85 +0x47
created by github.com/ethereum/go-ethereum/eth/downloader.New
        github.com/ethereum/go-ethereum/eth/downloader/downloader.go:244 +0x433

The spindown path is via

        case <-s.done:
            d.spindownStateSync(active, finished, timeout, peerDrop)
            return nil

And, is s.done is closed, that means the timers may or may not fire at all. This PR removes the s.done-check from the timer func, so it always triggers.

[karalabe]

Let's spin the dice again